Reorganize the "Canonical Built-ins" section of Explainer.md. (#413)

* Reorganize the "Canonical Built-ins" section of Explainer.md.

This moves the signatures for the canonical builtins out of prose paragraphs
and into tables, which highlights them and simplifies the prose.

This also adds "approximate signatures". The canonical ABI signatures are
useful when one is writing compilers or bindings generators or other tools,
while the approximate signatures are useful for people seeking to
understand what these builtins do at a conceptual level, before thinking
about how all the various values are lowered into `i32`s.

And, the approximate signatures in theory are independent of the ABI, so
they could be reused when new ABIs are added.

* Refactor event.poll and event.wait and types.

* Add documentation about the `repr<T>` type in the Canonical ABI.

* Split `stream.read` and `stream.write` into separate sections.

* Add the error-context to the Canonical ABI return type for `close-write`.

* Document `resource.new`/`.drop` validation.

* Change `complete`'s payload to `u32`.

* Document that stream.new returns a writable-stream.

* Desecribe how status values are lowered into the canonical abi.

* Describe status ABIs, merge read/write again.

* Document the `..1` and the "at most" behavior.

* Document the `option<error-context>` in the Canonical ABI.

* Document that `future.cancel-*` report at most 1 element.

---------

Co-authored-by: Luke Wagner <mail@lukewagner.name>
Co-authored-by: Lann <github-lann@lannbox.com>

Author: 3 people

design/mvp/Async.md

@@ -550,9 +550,9 @@ comes after:
 [Lift and Lower Definitions]: Explainer.md#canonical-definitions
 [Lifted]: Explainer.md#canonical-definitions
 [Canonical Built-in]: Explainer.md#canonical-built-ins
-[`task.return`]: Explainer.md#-async-built-ins
-[`task.wait`]: Explainer.md#-async-built-ins
-[`thread.spawn`]: Explainer.md#-threading-built-ins
+[`task.return`]: Explainer.md#-taskreturn
+[`task.wait`]: Explainer.md#-taskwait
+[`thread.spawn`]: Explainer.md#-threadspawn
 [ESM-integration]: Explainer.md#ESM-integration
 
 [Canonical ABI Explainer]: CanonicalABI.md